1. Technical Field
This invention relates generally to optical electrophotographic color copiers.
2. Background Art
Color copiers which make multiple projection exposures onto separate color separation image frames of a charged photoconductor are known. It is generally understood that the focal plane will change from one color separation to another, due in part to the different wave lengths of exposure light and further in part to optical characteristics of the filter elements (red, green, and blue, for example) used to create the different color exposures. If uncompensated for, these shifts in the focal plane between color separations will cause an out-of-focus condition at the photoconductor surface for one or more of the color exposures.
An out-of-focus condition in one color separation may decrease the exposure of very fine detail to the extent that color balance is affected. This is generally not a problem when copying reflection original documents because most documents being copied are made up of coarser images in which color balance is substantially unaffected by out-of-focus conditions.
However, the problem is aggravated in copiers adapted to reproduce images from photographic transparencies, which are often projected onto the photoconductor surface through a halftone screen to enhance tone scale. The screens themselves create the very fine dot pattern detail, even in the solid areas of the image. Breaking the image into a fine dot pattern causes color shift problems when one color separation is exposed more out-of-focus than another color separation. Since the dot pattern covers the entire image, color shifts are quite noticeable.
Of course, one solution would be to employ better optics which would focus all colors uniformly. However, such optics is costly, and would require extensive hardware changes to copiers already